The AM Forum
March 29, 2024, 03:38:43 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: 1 [2] 3   Go Down
  Print  
Author Topic: Viking 2 Modulator  (Read 40771 times)
0 Members and 1 Guest are viewing this topic.
WD8BIL
Contributing
Member
*
Offline Offline

Posts: 4409


« Reply #25 on: May 27, 2011, 01:48:45 PM »

Good deal, Jack.

And it's a gooder thing you're doing bringing the 'ol girl back to life!!!

Yes, I did switch to cap input on the power supply. It is now a pi filter with new caps at both ends. I have considered, from time to time, shoe horning in a seperate screen supply but the gain would be negligable so I usually settle on 'if it works don't fix it"!

One thing I did do to the plate and mod iron is pull the bell ends off and drill holes in them for increased ventilation. After 38 years I guess I can conclude it helped!!

 
Logged
The Slab Bacon
Member

Offline Offline

Posts: 3934



« Reply #26 on: May 27, 2011, 02:01:14 PM »

Jack,
        You wont be the first to do 6146s as modders. I did 2 apaches that way 5 or 6 years ago. Grin  Grin They work just fine. On one I even used a DX-100 mod tranny
(Made for a pair of 807s)

A valiant also uses 6146 for modders. 3 in the final modded by a pair.

If you want to keep it running in AB1 class, why not use a pair of 6550s, they're designed to run in either Class A or AB1 service. (Or a pair of the KT series bottles) With 600+V on the plates, they'll really swing the munky!!
Logged

"No is not an answer and failure is not an option!"
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #27 on: May 27, 2011, 02:18:46 PM »



Edit, I went to the shack, and pulled my old Viking I mod transformer out of the spider pit of iron. The secondary is a dual winding, so I suppose that offered flexibility should we want to use the modulator as a driver to some real tubes.

Anyway using a 1khz audio generator, and a scope, I measured the turns ratio. I put 8v P-P across the 807 side primary, and I get 5v P-P across the two secondaries in series. So 8/5 = 1.6 turns ratio, or 2.56:1 impedance ratio.

So lets say you tune up the Viking II to 250ma (plate plus screen currents) at 700V. The modulator load impedance will be 700/.25= 2800 ohms. Multiply this by 2.56 and the 807's will see 7168 ohms plate to plate. If the modulator tubes could swing the plate down to 100v (from a B+ of 700v), then each plate will swing from 100v to 1300v or 1200v peak. Across the whole primary, one tube will dip to 100v when the other peaks at 1300v. Convert that to RMS, take 1200 X .707 and we get 848 v AC rms. Since the load impedance is 7168 ohms, and we can put 846 volts RMS into that impedance, the power would be E^2/R or 846^2/7168 = 100 watts RMS.

So, the Viking II mod transformer with a 1.6:1 turns ratio, 700v B+, and the final loaded to 700v@250ma will allow 100 watts of audio at the clip point. This assumes the transformer does not saturate at that power level. If it does saturate, you can Taylor the low frequency response, or get the DC current off the secondary (Modified Heising).

Jim
WD5JKO




Logged
WD8BIL
Contributing
Member
*
Offline Offline

Posts: 4409


« Reply #28 on: May 27, 2011, 02:24:52 PM »

Thanks Jim.

That explains why the one in the Viking Bud has survived my abuse all these years!!!

The only thing the manual gives is the inductance.... 1.5 to 1.75 H.

And yes, there are hookup diagrams in the manual for using it to drive a higher power modulator.

Good work!
Logged
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #29 on: May 27, 2011, 04:21:18 PM »

That explains why the one in the Viking Bud has survived my abuse all these years!!!

Remember that core saturation and good insulation were not factored in.

So with the variables I came up  where we make 1200v peak across the mod transformer primary with a 700V B+ on BOTH the modulator and the RF final, when we factor in the turns ratio of 1.6 to get the modulated B+ peak voltage: 1200/1.6 = 750v. So 750/700 = 1.06, or 106% modulation capability. If the turns ratio were more the ideal ratio of 1.4, then we could modulate to 122%. Take the Apache mod transformer at 1.9:1, and we then have only 84% capable modulation...Hence the need of the Turbo Mod.

Those calculations I gave were a first pass with variables picked out of the air. So try varying the parameters such as the plate voltage minimum where I selected 100v (EB min). A typical multi element tube will start to go non linear as the plate swing goes lower than the screen voltage. Here the beam power tubes have an edge over the Pentodes, and both have an edge over the Tetrodes. Remember the KT series of tubes where KT stands for "Kinkless Tetrode"?

Tubes like the 807 with a screen voltage of 300v or so, will show a big difference with EB min in class Ab1 versus Ab2. Other tubes like sweep tubes, and even the 6146 where the G2 voltage might be 175V or so, will swing further in class Ab1. This is all generalization though, and there are good tubes in each category. Lots of variables to juggle.. :-)

Jim
WD5JKO 
Logged
KZ5A
Member

Offline Offline

Posts: 94

Vikings Rule!


« Reply #30 on: May 27, 2011, 06:17:02 PM »

This thread is getting to be highly educational. Grin Grin

Jim, thanks for the detective work and the example.  I put all that into a spreadsheet and played with it a bit.  Assuming a efficiency of 59% for the class C  stage, the RF PO and the mod PO match closely and track with various final plate voltages and loadings, maintaining a 1 to 1 ratio.  Do mod transformer losses figure into that ratio significantly?

Buddy, I'd be interested to know how much HV the Bud has with the variac set to a nominal 115 VAC, idle and loaded.  I'm curious about how much difference there is in real life as opposed to theoretically Roll Eyes Roll Eyes between choke input and cap input.   Also how much cap are you using?

 

73 Jack KZ5A
Logged

73 Jack KZ5A
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #31 on: May 27, 2011, 10:16:16 PM »

Do mod transformer losses figure into that ratio significantly?

   Jack, You could factor in the transformer winding resistances. On my Viking I transformer, the primary is 101 ohms each side of centertap. Each secondary is 37 ohms, or 74 ohms total. While at it, I put my Beckman DM27XT on the inductance range, and I get 3H on each side of the primary, and each secondary is 1.2H. This meter runs at 1 Khz or thereabouts and does not take into account reduced inductance from DC bias of the core when the secondary has 1/4 amp flowing through it.

  I would think a good transformer would be about 95% efficient, a SWAG.

Jim
WD5JKO
Logged
w4bfs
W4 Beans For Supper
Contributing
Member
*
Offline Offline

Posts: 1433


more inpoot often yields more outpoot


« Reply #32 on: May 27, 2011, 11:32:12 PM »

6146's are not the best choice for modders BUT if you would add some negative feedback to the circuit you could lower the plate resistance from several thousands of Ohms to several hundred Ohms ... this would lower peak power distortion ....there are several ways to do this ....consult old handbooks or iffin you are Very lucky Tim just might speak
Logged

Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #33 on: May 28, 2011, 08:57:53 AM »

6146's are not the best choice for modders BUT if you would add some negative feedback to the circuit you could lower the plate resistance from several thousands of Ohms to several hundred Ohms ... this would lower peak power distortion ....there are several ways to do this ....consult old handbooks or iffin you are Very lucky Tim just might speak

   Well I am not Tim, but maybe we can smoke him out.  Smiley

Looking around the net, the 6146 has not been embraced by audio folks. Here are a few reasons:

* They don't like the plate cap for safety reasons
* Susceptible to VHF parasitics, need grid stopper resistor in AB1
* Need higher peak grid swing compared to high Gm audio tubes like 6550. This requires a low distortion driver at high peak levels. This eliminates simply driving with a hot cathode phase inverter like the triode side of a 7199, so therefore another stage is required, or transformer drive is needed.
* These tubes like a low Z path to fixed bias, 100K max. Other more popular tubes might get away with 270K to 470K.
* Found many references to a stiff G2 voltage requirement, and at ~ 200v. This means Ultra linear operation with screen taps on output transformer is out. The Acrosound folks got around this with the TO-350 using tertiary screen windings. Still using conventional P-P with a regulated G2 supply should work fine.
* Matched sets of 6146 are not available. You need to match them yourself, or have independent tube bias adjustment.

Here is a 6146 audio tube amp that seems to overcome the specific 6146 needs:

http://www.bonavolta.ch/hobby/en/audio/6146.htm

Also the legendary Acrosound ciruit:
(page 11)
http://www.clarisonus.com/Archives/Trans/Acro55.pdf

Jim
WD5JKO
Logged
KZ5A
Member

Offline Offline

Posts: 94

Vikings Rule!


« Reply #34 on: May 28, 2011, 11:32:50 AM »

Jim,

Welllll...... I'm not an "audio guy" Roll Eyes

Great info, so looking at these points one at a time.

Quote
* They don't like the plate cap for safety reasons

I like them, actually prefer them to keep less HV routed around under the chassis.

Quote
* Susceptible to VHF parasitics, need grid stopper resistor in AB1

I noticed that several mod schematiics have something on the order of a 100 ohm R at the grid connection, which I included in my plan.  Is that what you are referring to???

Quote
* Need higher peak grid swing compared to high Gm audio tubes like 6550. This requires a low distortion driver at high peak levels. This eliminates simply driving with a hot cathode phase inverter like the triode side of a 7199, so therefore another stage is required, or transformer drive is needed

The K6AD 6AU6/6AU6 circuit I'm using is alleged to work well with the 807's which have only slightly lower G-G drive voltage requirements than 6146's per the RCA TT-5 manual.  So I think I'm OK there.

Quote
* These tubes like a low Z path to fixed bias, 100K max. Other more popular tubes might get away with 270K to 470K.

I'm a little vague on this one, my circuit only has a 100 ohm grid R and a 10K R between the grid and the bias supply filter cap.  I don't understand how one would get to a 100k between the grid and the bias supply so I''m probably missing something here.

Quote
* Found many references to a stiff G2 voltage requirement, and at ~ 200v. This means Ultra linear operation with screen taps on output transformer is out. The Acrosound folks got around this with the TO-350 using tertiary screen windings. Still using conventional P-P with a regulated G2 supply should work fine.

Regulating the mod G2 voltage has always been part of the plan regardless of tube selection.

Quote
* Matched sets of 6146 are not available. You need to match them yourself, or have independent tube bias adjustment

I picked up a dozen 6146Ws ("B" type) so time back and found 3 ~closely matched pairs among them.  So I'm good there.  I had already provided for independent cathode current measurement by putting a 10 ohm R in each cathode.   Hadn't thought about providing independent bias adjustment but that sounds like a good idea and shouldn't be too hard.


WD4BFS

Negative feedback is also part of the plan.  I've seen several implementations and haven't really settled on one.   This might one of those things that has to wait to be evaluated in the working TX.  In any case I plan on making this adjustable.


All,

I think it is time to commit all I've learned here to paper and produce a schematic that I can scan and post for discussion.   Got the old clearprint paper and schematic templates in front of me now.

Thanks to all.

73 Jack KZ5A


Logged

73 Jack KZ5A
KZ5A
Member

Offline Offline

Posts: 94

Vikings Rule!


« Reply #35 on: May 29, 2011, 11:45:26 AM »

Ok,

Here is the Schematic as it stands today.  I haven't calculated some of the PS values and some caps and zeners are "composite" objects.  For instance the HV filter cap is actually 3 x 250uf @ 450VDC.

Comments, suggestions, and criticisms all welcomed.

73 Jack KZ5A
Logged

73 Jack KZ5A
KZ5A
Member

Offline Offline

Posts: 94

Vikings Rule!


« Reply #36 on: May 29, 2011, 11:48:24 AM »

Forgot to attach the drawing, obviously need more coffee this morning.....

73 Jack


* Viking 2 KZ5A Mods.jpg (2284.42 KB, 3561x2487 - viewed 640 times.)
Logged

73 Jack KZ5A
WD8BIL
Contributing
Member
*
Offline Offline

Posts: 4409


« Reply #37 on: May 29, 2011, 12:47:11 PM »

Jack,

At nominal line (117 here) Static (no drive) voltage is @775VDC. At 100 watts out it drops to 754VDC.
It's been a looooonngg time since this thing was stock but ifn I remember right the unloaded plate was around 700- 710.
Logged
KZ5A
Member

Offline Offline

Posts: 94

Vikings Rule!


« Reply #38 on: May 29, 2011, 01:39:43 PM »

Buddy,

Looks like about 10%, some of the literature I've read suggested it would be more like 30%.

Did you keep the 20K HV bleeder in the circuit?

73 Jack
Logged

73 Jack KZ5A
w4bfs
W4 Beans For Supper
Contributing
Member
*
Offline Offline

Posts: 1433


more inpoot often yields more outpoot


« Reply #39 on: May 29, 2011, 02:09:43 PM »

the bias adjust will not work as drawn .... the secondary of the transformer would need to be ac coupled with a blocking cap on at least one of the secondary legs ....this is ok unless you are planning on driving into AB2 which would be dicey with 6146 anyway .... I would not use a balancing bias shift as drawn but a voltage divider then to 100k grid resistors ....this is to cut down on coupling cap size into grids ....hope this helps andmakes sense
Logged

Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #40 on: May 29, 2011, 04:25:51 PM »

the bias adjust will not work as drawn .... the secondary of the transformer would need to be ac coupled with a blocking cap on at least one of the secondary legs ....this is ok unless you are planning on driving into AB2 which would be dicey with 6146 anyway .... I would not use a balancing bias shift as drawn but a voltage divider then to 100k grid resistors ....this is to cut down on coupling cap size into grids ....hope this helps andmakes sense

Good Stuff!

   I was thinking the same thing, then I realized that pot wiper sets the AC center of the drive instead of change the bias voltage. This is because the driver transformer has no CT. So this acts as an AC balance.

   To vary the bias, id'd just put a pot across the 75V zener, and bypass the wiper. This provides 0-> -75v mod bias instead of -50v fixed.

  Adjusting the bias, and adjusting the AC balance are good things. Then adjusting each tube bias independently throws a wrench into the circuit. Would be easier to just match the 6146's.

More to come!

Jim
WD5JKO
Logged
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #41 on: May 29, 2011, 08:43:51 PM »


Jack,

   I suggest you zero in on the 6AU6 driver stage. Here we will have resistive and core losses in the interstage transformer, and your two 10K's and 5K pot place a 25K load on the transformer. So just from the 25K load, with a bias on the 6146's of -50V, and full drive to the limits of Ab1 operation, each grid will swing from -100 to 0 volts; grid to grid you got 100v peak, or 70.7 v AC rms. Take P=E^2/R, 71^2/25000, and we have 200mw. The transformer losses might match that as well, so lets figure your going to need 1/2 watt of drive.

  To get 1/2 watt from a single ended class A amplifier with transformer coupling, the efficiency if everything is perfect is around 50% at the clip point. More than likely you will be at ~ 25%. This means your 6AU6 DC operating point needs to be somewhere between 1 to 2 watts. So this means you might need a lot more plate current than your circuit currently has.

  I suggest you look at the single ended pentode load line tutorial by Patrick Turner:

http://www.turneraudio.com.au/loadmatching1sebeamtetrodes.htm

and here is a 6AU6 data sheet:

http://www.r-type.org/pdfs/6au6-1.pdf

Note, the Heathkit Apache using EL-34's as modulators in Ab1, use a triode connected 12BY7 as an audio driver.

I think the 6AU6 might work, but to do so you got to crank up the juice to it. Knowing what the G2 voltage is will be helpful. Maybe make that determined by a voltage divider instead of just a series dropping resistor.

Jim
WD5JKO
Logged
WD8BIL
Contributing
Member
*
Offline Offline

Posts: 4409


« Reply #42 on: May 29, 2011, 08:57:04 PM »

Quote
Looks like about 10%, some of the literature I've read suggested it would be more like 30%.

I was a bit surprised too, Jack, till I measured voltages. At the first cap there's just over 800 loaded so that Johnson choke has quite a bit of resistance. Yes the 20K bleeder is still in there.
Logged
KZ5A
Member

Offline Offline

Posts: 94

Vikings Rule!


« Reply #43 on: May 29, 2011, 09:19:31 PM »

Revision 1.1 attached.  Fixed a couple of boo-boo's, added a few things, and tweaked a few values.

Quote
the secondary of the transformer would need to be ac coupled with a blocking cap

Why?  Neither the original design or any of the mods I've looked at do so.  Or maybe I'm misunderstanding what you are saying?

The two 10K R's and the 5K pot across the secondary are intended to do two things.  One is to apply bias in the absence of a center tap on this particular transformer (Hammond T-124B), the other is to act as the load for the 6AU6 driver.  This is strictly an AB1 design, so the grids not present a load to the driver.  My thinking was that the driver would play better if it was working into a nice matched resistive load.  Or maybe that's just an RF thing?

The pot in the center was intended to be used to balance the tube idle currents, I hadn't thought about the effect on the AC drive balance.  But after thinking about it some more, it probably will not effect the relative bias due to the lack of grid current.   I can definitely see how it would effect the AC balance however, since it is tied directly to the 150 uf bias supply filter cap and the wiper will have a low Z to ground. 

So I guess that pot is a good thing, by accident....even a blind hog finds an acorn occasionally. Cool Cool

The final value of these R's are to be determined in testing later, these values are just SWAG starting points.  Same with the mic transformer load resistor.

According to the RCA TT-5 manual and my other AB1 6146 rigs, the bias for a 6146 in AB1 is -50V.  I intend to "adjust" this in testing by changing the Zener as required.   A pot across the 75V bias would work fine for the AB1 mod bias, and save some soldering iron time, probably put that in the next revision.

Jim,

I was listening to 3890 this AM and heard you mention that you are using PC software to do your audio testing.  Wonder what you are using and your impressions of it.   The station PC has tons of power (4 cores@3200mhz) and a vacant slot so that's probably going to be the best approach to making the distortion measurements needed to tweak this design.  All the hardware audio distortion meters I've looked at have been a bit too pricey.  I could put a SB Audigy Z2 in it or just "borrow" the EMU 0202 from the LP-PAN setup for testing.

Audio test equipment is at the top of my Ham-Com need list.  I'll be looking for a scope, and an audio generator.

Your last post came in while dotting the I's and crossing the T's on this one.   The tutorial looks good, it will take me a bit to digest it.

Other than the change to 6146s, this audio mod is largely the K6AD mod from ER #110/111, which allegedly works well.  One potential difficulty arises from the greater voltage swing required by the 6146's relative to the 807's in the K6AD circuit.   The 6AU6 driver circuit is supposed to provide a 300V swing on the primary side,  with a 3/1 turns ratio,  100V on the mod side.   Not much room for losses.



73 Jack KZ5A

 



* Viking 2 KZ5A Mods rev 1.jpg (3163 KB, 4000x3000 - viewed 604 times.)
Logged

73 Jack KZ5A
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #44 on: May 29, 2011, 09:44:54 PM »

Jim,
I was listening to 3890 this AM and heard you mention that you are using PC software to do your audio testing.  Wonder what you are using and your impressions of it.   The station PC has tons of power (4 cores@3200mhz) and a vacant slot so that's probably going to be the best approach to making the distortion measurements needed to tweak this design.  All the hardware audio distortion meters I've looked at have been a bit too pricey.  I could put a SB Audigy Z2 in it or just "borrow" the EMU 0202 from the LP-PAN setup for testing.

Jack,

So how did my Retro75 + 813 sound when hitting 150% positive peaks? :-)

  I purchased the Dr. Jordan audio test software, the "educational" version. This version is somewhat neutered, but still does a whole bunch of stuff. For $50 or so ($/Euro conversion) it comes with a CD, and USB Dongle. It is registered to whoever buys it by name. Those Germans are efficient.

Look here about that software:
http://www.tubelab.com/testing.htm

There are some free evaluation downloads. I like the audio generator. You can run two at once for two tone output.

http://www.dr-jordan-design.de/Downloads.htm

Also see attached schematic for a higher powered 6AU6 driver where we get rid of the interstage transformer coupling issue. Food for thought. Looks like they run about 12ma plate current on the 6AU6..

Jim
WD5JKO

* psa_el84_reva.pdf (33.77 KB - downloaded 236 times.)
Logged
KE6DF
Member

Offline Offline

Posts: 784


WWW
« Reply #45 on: May 29, 2011, 10:09:48 PM »

This has probably already been covered and ruled out for some reason, but couldn't you just replace the 6AU6 with a 12B4?

You would then have a way better driver tube.

You would need to replace the 7 pin miniature socket with a 9 pin miniature, but the tube would fit in the same space. Seems like you could keep the driver transformer.

Perhaps there wouldn't be enough gain?Huh

Dave
Logged

KZ5A
Member

Offline Offline

Posts: 94

Vikings Rule!


« Reply #46 on: May 29, 2011, 10:35:48 PM »

Jim,

Sounded really good, looked good on the pan-adapter as well.   

I've got to the point where I can recognize several folks without listening, just from looking at the pan-adapter.  You can see everyone's signal width and modulation patterns in great detail.  Same display as the Flex.

I'm not stuck on the 6AU6 driver,

0934 Z just heard you CQ on 7290 but my barefoot K3 PW isn't making it to Round Rock!!

73 Jack
Logged

73 Jack KZ5A
KZ5A
Member

Offline Offline

Posts: 94

Vikings Rule!


« Reply #47 on: May 29, 2011, 10:37:08 PM »

Oops, 0934 local

73 Jack
Logged

73 Jack KZ5A
KZ5A
Member

Offline Offline

Posts: 94

Vikings Rule!


« Reply #48 on: May 29, 2011, 11:58:32 PM »

Anyway, back to the 6AU6 driver...

The 1960 ARRL Handbook has a 6146 AB1 modulator alleged to make 120W with 750V.  One of my other sources of inspiration.  It uses a 6C4 as the driver running it off of the 200V supply shared with the 6146 screens.   Same 3 to 1 driver transformer ratio, same 3.5W Max Pd in class A.

The K6AD first 6AU6 stage has less gain and much better fidelity than the Johnson design.  The K6AD second 6AU6 was changed to pentode connection to "make up" for the lower first stage gain.  So I'm thinking putting a 6C4 in place of the 6AU6 driver, without doing the 12AX7, is probably a non-starter.

A 6AQ5 would have higher max Pd, but I don't know about more gain.

BTW The T-124B measures about 400 ohms on the primary and 1500 across the secondary.

Time to QRT for the day.

73 Jack KZ5A
Logged

73 Jack KZ5A
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #49 on: May 30, 2011, 12:52:25 AM »

Anyway, back to the 6AU6 driver...

The 1960 ARRL Handbook has a 6146 AB1 modulator alleged to make 120W with 750V.  One of my other sources of inspiration.  It uses a 6C4 as the driver running it off of the 200V supply shared with the 6146 screens.   Same 3 to 1 driver transformer ratio, same 3.5W Max Pd in class A.

  Good Deal. Just poking around and looking at that. The 6C4 is a good choice for sure, but low gain. For kicks I dug out the 6AU6 triode connected characteristics (grid 2, grid 3 tied to plate). Looks like the 6AU6 in triode mode has higher Gm (4800 versus 2200 uMho), and higher Mu (36 versus 17) than the 6C4. The plate resistances are about the same. Looks like the 6C4 would need ~ 2X the AC drive for the same plate swing..

http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_1/6AU6-A.PDF
The last chart for 6AU6 shows triode connection

http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_1/6C4.PDF


Could make the Viking II audio input a line level input...Don't have to add that 12AX7 :-)

Good Night OM

Jim
WD5JKO

Logged
Pages: 1 [2] 3   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.052 seconds with 18 queries.